182 CHEMICAL ENGINEERING
of this pipe, a blast gate is inserted, while at the far end the measuring orifice, or a header containing a number of small orifices, is bolted on.
The orifice is usually selected to pass a quantity about 35 per cent greater than the rated quantity. The following rough formulas are frequently used: Q = 109cZ2\/P = 2ld*\/i, where Q = cubic feet of air per minute; d = diameter of orifice, inches; P = pressure above atmosphere, pounds per square inch, and i ~ inches of water. Readings are generally taken at about six or seven different loads, the readings taken being the inlet temperature, the discharge pressure, the orifice pressure and temperature and the revolutions per minute. The discharge pressure is taken with an impact tube inserted into the discharge pipe about midway between the compressor and the blast gate, pointing upstream and reaching a little more than half way into the pipe. A second impact tube nearer the blast gate is desirable. The orifice pressure is taken with an impact tube placed opposite the center of the orifice at a distance from the plane of the orifice equal to about one-fifth of the orifice diameter. The orifice temperature is taken in the straight part of the pipe before the orifice. The bare bulb of the thermometer should protrude for about 2 in. into the stream of air, the air velocity rarely exceeding 60 or 70 ft. per second.
The efficiency of a centrifugal compressor is the ratio of the theoretical power, adiabatic or isothermal, corresponding to the rated quantity and pressure, divided by the power delivered to the compressor shaft. There is, however, considerable room for controversy and misunderstanding in this connection, owing to the difficulty of measuring accurately the output of the driver. The input of the driver, however, is generally easily determinable. It is therefore customary to specify that the input of the driver per 100 cu. ft. of air delivered by the compressor shall not exceed so many kilowatt-hours, or so many pounds of steam, the steam conditions being those of the customer's plant.
Commercial sizes of the single-stage compressors have pressure ratings from 0.75 to 4 Ib. per square inch and capacities from a lower limit of 500 cu. ft. per minute to a higher limit which ranges from 12,000 cu. ft. at 0.75 Ib. pressure down to 3,000 cu. ft. at 4 Ib. pressure. The multi-stage compressors are built of the following sizes:
Cubic feet per minute.............................. 4,500 9,000 16,000 25,000 40,000 50,000
Pressure, pounds per square inch.................... 6-35 6-25 6-25 12-30 12-30 12-30
Multi-stage compressors have also been built for pressures as high as 150 Ib. per square inch, the volumes being comparatively small.
With a driver allowing speed variation, like a steam turbine or direct-current motor a centrifugal compressor of a given rating will operate with a number of different combinations of pressure and volume without excessive loss in efficiency. The different rated pressures of multi-stage compressors are usually obtained by the use of a different number of stages, all stages in a given size of machine being similar. A given compressor may, however, operate successfully between, say, 15 and 30 Ib. per square inch, if the driver will allow of the necessary speed variation.
Without any special governing device a centrifugal compressor driven at constant speed will maintain a substantially constant pressure, regardless of the volume of air delivered. Should there be temporarily no demand for air while the compressor is being driven at full speed, the casing will become slightly hotter, but otherwise the machine will not be subjected to any particular strains. The power required by a centrifugal compressor between half-load and, say, 25 per cent overload varies approximately as the load. The following table shows the usual variation of pressure and power with the volume handled by a single-stage compressor at constant speed.
For blast-furnace purposes, where a constant quantity of air is to be delivered against varying pressures, a constant-volume governor has been developed.